The present invention relates to a process for producing sucrose from sugar cane.
The production of cane sugar for human consumption generally comprises two distinct operations, namely the production of raw sugar and the production of refined sugar, which are often carried out in separate locations. Production of raw sugar typically takes place at a sugar mill, which is usually located in or near sugar cane fields. In the mill, sugar cane stalks are chopped or shredded into pieces and the pieces are crushed in a series of mills in order to remove the juice. The juice from the first set of roller mills is referred to as xe2x80x9cfirst juice,xe2x80x9d while the total juice from all the roller mills in the process is referred to as xe2x80x9cmixed juice.xe2x80x9d The juice is normally limed, deaerated and clarified (i.e., removal of suspended solids, usually by sedimentation). The clarified stream is referred to as xe2x80x9cclarified juice.xe2x80x9d The juice is then evaporated to a thick syrup (known as xe2x80x9cevaporated juicexe2x80x9d), and crystallized in a vacuum pan. The xe2x80x9cmassecuitexe2x80x9d (i.e., mixture of sugar syrup and crystals) produced in the vacuum pan is stirred in a crystallizer, and the mother syrup is spun off from the raw sugar crystals in a centrifugal separator. The solid sugar in the centrifugal basket is washed with water to remove remaining syrup. The solid crystalline product is termed xe2x80x9craw sugar.xe2x80x9d The mother liquor is then crystallized a further two times to obtain a greater yield of sugar, and the mother liquor is molasses, which can be sold for fermentation or as an animal feed.
Depending on the exact nature of the process steps and conditions used in the sugar mill, the raw sugar product can be made more or less pure. A more highly purified mill product is sometimes referred to as xe2x80x9cMill Whitexe2x80x9d or xe2x80x9cPlantation Whitexe2x80x9d sugar. The production of these sugars requires sulphitation, before or after clarification, using SO2 gas. It usually requires a second clarification step, usually at the syrup stage and sometimes a second sulphitation step. In nearly all cases the ash content of this sugar is much higher, perhaps by more than four times, than that of refined white sugar. Although these particular mill products can be sold for human consumption without further processing in some instances, generally raw sugar must be further refined before it reaches a commercially acceptable level of purity, particularly for subsequent use by food and drink manufacturers.
Therefore, the raw sugar from a mill is usually transported to a sugar refinery for further processing. In a conventional cane sugar refining process, the raw sugar is first washed and centrifuged to remove adherent syrup, and the xe2x80x9caffined sugarxe2x80x9d thus produced is dissolved in water as xe2x80x9cmelter liquor.xe2x80x9d The syrup removed from the surface of the raw sugar is known as xe2x80x9caffination syrupxe2x80x9d and is broadly similar in composition to the mother syrup from the raw sugar crystallization. The affination syrup is processed through vacuum pans, crystallizers and centrifugal separators similar to those used for the production of raw sugar, to recover an impure crystalline sugar product which has approximately the same composition as raw sugar. This recovered sugar product is dissolved in water, along with the affined raw sugar, to make melter liquor. Thus, the treatment of affination syrup in the recovery house of the refinery is somewhat similar to the production of raw sugar from evaporated juice.
The melter liquor is then purified, generally by the successive steps of clarification (also referred to as xe2x80x9cdefecationxe2x80x9d) and decolorization, and the resulting xe2x80x9cfine liquorxe2x80x9d is crystallized to give refined sugar. The clarification step usually involves forming an inorganic precipitate in the liquor, and removing the precipitate and along with it insoluble and colloidal impurities which were present in the melter liquor. In one of the clarification processes commonly used for melter liquor, termed xe2x80x9cphosphatation,xe2x80x9d the inorganic precipitate is calcium phosphate, normally formed by the addition of lime and phosphoric acid to the liquor. The calcium phosphate precipitate is usually removed from the liquor by flotation, in association with air bubbles. Other clarification processes, termed carbonation (or carbonatation) processes, involve adding lime and carbon dioxide to the liquor, and produce calcium carbonate precipitate. This is removed by filtration, usually under pressure.
The geographical separation of cane sugar milling and refining operations is a common feature of the industry. It is not practical to build a refinery at the site of every cane sugar mill, due to the relatively large capital cost of conventional refining process equipment.
The juice produced in a cane sugar mill typically has a color of about 14,000 icu, and conventional mill technology can process this to raw sugar with a whole color of 2000 to 5000 icu, and a well affined color of 400-800 icu. It is very difficult to produce white sugar of less than 80 icu in one crystallization in a mill because of the extremely high colors of the starting material, and because it is difficult to filter cane juice or syrup. After a crystallization at the mill, a significant portion of colored materials are concentrated in the raw sugar crystals, and when the raw sugar is refined a high degree of decolorization is required in order to produce white sugar.
One process that has been used in an attempt to overcome this problem is referred to as the Java process. A juice stream in a cane sugar mill is treated with an excess of lime, usually at least equal to about 10% by weight of the sugar in the juice. Excess lime is removed with carbon dioxide. This process evolved into the deHaan process, which used milk of lime and carbonation, at 55xc2x0 C. The deHaan process used multiple incremental additions of milk of lime followed by carbonation. These processes did improve the color of the crystallized sugar product from the mill, but the very large amount of lime required in order to achieve good filtration made the processes economically undesirable, as well as needing a large amount of filtration equipment, and producing a large amount of material that would need to be disposed of, giving environmental problems.
Attempts have been made in the past to incorporate membrane filtration into the A processing of cane sugar. However, such attempts have generally used membrane filtration as supplement to conventional clarification steps using lime. Therefore, the equipment cost of such proposed processes has tended to be relatively high.
In addition, the roller mills used in cane processing are very large and expensive, and typically require frequent maintenance. A cane sugar process that wholly or partially eliminates the need for such equipment could offer substantial cost savings.
There is a need for improved cane sugar processes that would allow production of a highly purified product using fewer crystallizations, and preferably in a single plant, rather than in separate sugar mills and refineries, in order to reduce the cost and simplify the processing of cane sugar for human consumption.
The present invention relates to a process for producing sugar from cane that includes the steps of: (a) grinding sugar cane or pieces thereof into pulp; (b) mechanically separating juice from the pulp; and (c) membrane filtering the separated juice, producing a retentate and a permeate. In step (a), the cane is ground into pulp comprising particles having an average fiber length of considerably less than twenty millimeters, preferably less than about ten millimeters, most preferably less than about 5 mm. The average fiber diameter will be less than about 500 microns and preferably less than about 200 microns. Sugar cane consists of a hard fibrous rind which surrounds a softer pith. When milled the rind forms long fibers whereas the pith tends to be broken down in size more easily. Grinding to a small size allows more complete extraction of sucrose from the bagasse, increasing extraction and the yield of the factory. The sugar produced by this process is white or low color sugar (e.g., a color no greater than about 35 icu).
The mechanical separation of juice from cane pieces can be done, for example, by filtration or centrifugation. It is preferred that water be added to the cane pieces during or prior to centrifugation, either as pure water or as juice that also contains some sucrose.
In addition it is preferred to adjust the pH of the separated juice to at least about 7 prior to membrane filtration, more preferably to at least about 7.5. This pH adjustment can be achieved by adding various agents, but lime or sodium hydroxide are especially preferred. Optionally the separated juice also can be contacted with an agent selected from the group consisting of sulfur dioxide, sulfite salts, bisulfite salts, and mixtures thereof.
A variety of membrane types and filtration conditions can be used. Microfiltration, ultrafiltration, and nanofiltration membranes are examples of types of membranes that are suitable for use in this process.
Grinding the cane to pieces with a fiber length preferably less than about 10 mm, most preferably less than about 5 mm, and a fiber diameter of about 200 microns or less can allow the release of more impurities than in conventional milling. Often these impurities can interfere with subsequent purification, and make the extraction of sucrose by crystallization difficult. The use of a membrane allows removal of many of these impurities, allowing more straightforward processing to white sugar.
After the membrane filtration, the permeate can be concentrated and sucrose crystallized therefrom. Although additional purification steps can be used between the membrane filtration and the concentration/evaporation, in one embodiment of the process no further purification of the permeate occurs after membrane filtration and prior to crystallization. It is particularly preferred that the juice or the permeate is not subjected to carbonation, which involves the addition of lime and carbon dioxide.
One specific embodiment of the invention is a process that includes the steps of: (a) grinding sugar cane or pieces thereof into pulp that comprises particles having an average length of less than about 5 mm and an average diameter of about 200 microns or less; (b) adding water to the pulp; (c) mechanically separating juice from the pulp; (d) adjusting the pH of the juice to at least about 7.0; (e) membrane filtering the juice through a membrane having a molecular weight cutoff between about 1,000-10,000, producing a retentate and a permeate; and (f) concentrating the permeate and crystallizing sucrose therefrom. Carbonation of the juice or the permeate is not carried out in this embodiment of the invention.
Sugar produced in accordance with the present invention is low in ash (considerably lower than plantation white sugar), low in polysaccharides and other floc-forming impurities, and can meet a refined white sugar specification.
The process of the present invention has many advantages over the conventional cane sugar processes that use liming and carbonation. For instance, this process can achieve a higher extraction of sucrose than prior processes. Grinding the cane to a greater degree improves the ease of extraction of sugar from the cane, as it difflises more easily from the finely ground particles.
Another advantage is the reduction in required process steps and equipment. The process of the present invention can produce white sugar directly at a cane mill without the need for refining at a separate facility. Alternatively, the process can produce raw sugar that has very low color and thus requires less equipment and fewer processing stages in the refinery.
The short residence time of the process combined with heating to a lower temperature eliminates the production of materials such as extra color and gelatinized starch that make subsequent purification by the conventional process more difficult. The process eliminates the extensive use of lime, and the disposal of carbonate cake resulting in a drastic reduction of waste products that cause environmental pollution. The conventional process produces a filter cake that comprises products of the liming process and impurities removed from the juice. The proposed process completely eliminates the need for disposal of such materials.